For any integrated circuit technology, elements, devices, and cells must function with reproducible and uniform behavior. But they cannot all be tested individually; this would require an excessive number of input and output contacts. This is particularly important for a technology at the Very-Large-Scale Integration (VLSI) level, with tens of thousands of elements and beyond on a single chip. Ultrafast superconducting digital circuit technology, such as that based on rapid-single-flux-quantum logic (RSFQ), is now developing VLSI circuits, for which diagnostic testing is critical for further scaling to even higher density circuits with millions of elements per chip. RSFQ logic is based on signals comprising a time series of single-flux-quantum (SFQ) voltage pulses, each pulse comprising an integrated voltage over time of a single magnetic flux quantum Φ0=h/2e=2.07 mV-ps, where typically the pulse height is ˜2 mV and the pulse width is ˜1 ps. Because of the very narrow pulsewidth, RSFQ circuits are known to operate at very high clock speeds up to about 100 GHz, with extremely low power dissipation. They do, however, require cooling to deep cryogenic temperatures for operation, below the superconducting critical temperature Tc of the superconducting material. The most common superconducting material for RSFQ integrated circuits is niobium, with Tc=9.2 K.
One general approach to diagnostic testing is to fabricate an array of nominally identical devices, where different devices in the array may be appropriately activated and measured sequentially, using a minimal number of input and output lines. This is similar in concept to arrays that are used for memories or imaging, where selection techniques may be used to read out a given element in the array. One type of selector is a multiplexer (see, for example, en.wikipedia.org/wiki/Multiplexer), where a plurality of inputs are sent into the multiplexer, and only a single output is selected. A multiplexer may comprised of a plurality of selectable switches.
Superconducting Multiplexers and Demultiplexers are known in prior art superconducting electronics technology. See, for example, U.S. Pat. No. 5,982,219 (“Asynchronous Dual-Rail Demultiplexer Employing Josephson Junctions”, invented by A. Kirichenko, issued Nov. 9, 1999); and U.S. Pat. No. 8,611,974 (“Systems, Methods, and Apparatus for Superconducting Demultiplexer Circuits”, invented by F. Maibaum, et al., issued Dec. 17, 2013), each of which is expressly incorporated herein by reference in its entirety. Superconducting digital switches and switch arrays are also known in the prior art. See, for example, U.S. Pat. No. 7,362,125 (“Digital routing switch matrix for digitized radio frequency signals”, invented by D. Gupta and A. Kirichenko, issued Apr. 22, 2008), expressly incorporated herein by reference in its entirety. See also:    A. Kirichenko, “High-speed asynchronous data multiplexing/demultiplexing”, IEEE Transactions on Applied Superconductivity, vol. 9, no. 2, pp. 4046-4048, June 1999;    D. L. Miller, et al., “Single-flux-quantum demultiplexer”, IEEE Transactions on Applied Superconductivity, vol. 7, no. 2, pp. 2690-2692, June 1997;    L. Zheng, et al., “RSFQ multiplexer and demultiplexer”, IEEE Transactions on Applied Superconductivity, vol. 9, no. 2, pp. 3310-3313, June 1999; and    L. Zheng, et al., “50 GHz multiplexer and demultiplexer designs with on-chip testing”, IEICE Transactions on Electronics, vol. E85-C, no. 3, pp. 621-624, March 2002;
each of which is expressly incorporated herein by reference in its entirety.
One type of superconducting output device in the prior art is the Superconducting Quantum Interference Device, or SQUID, see for example en.wikipedia.org/wiki/SQUID. A SQUID (sometimes called a DC SQUID, although it is not limited to DC applications) comprises two Josephson junctions and an inductive loop, and essentially acts as a sensitive transducer of magnetic flux Φ (coupled into the loop) to output voltage V. A SQUID actually generates a time series of SFQ pulses, with frequency f=V/Φ0 (corresponding to a pulse rate of 483 GHz/mV), so it can also be used as a digitizer. Arrays of SQUID outputs with multiplexers have been previously disclosed. See U.S. Pat. No. 5,355,085 (Y. Igarashi et al., “Multichannel SQUID flux meter with multiplexed SQUID sensors”, issued Oct. 11, 1994); U.S. Pat. No. 8,593,141 (M. Radparvar and A. Kadin, “Magnetic resonance system and method employing a digital SQUID”, issued Nov. 26, 2013), each of which is expressly incorporated herein by reference in its entirety.
See also:    A. M. Kadin, et al., “Superconducting digital multiplexers for sensor arrays”, Proc. Int. Conf. on Thermal Detectors (TDW'03), June 2003, available online at ssed.gsfc.nasa.gov/tdw03/proceedings/docs/session_4/Kadin.pdf;    J. Chervenak, et al., “Superconducting multiplexer for arrays of transition edge sensors”, Applied Physics Letters, vol. 74, pp. 4043-4045, June 1999; and    A. Inamdar, J. Ren, and D. Amaro, “Improved Model-to-Hardware Correlation for Superconductor Integrated Circuits”, IEEE Trans. Appl. Supercond., vol. 25, no. 3, June 2015, article 1300308,
each of which is expressly incorporated herein by reference in its entirety.